Filter Writing Guides and Examples
http://www.antagonism.org/mail/folder-creation-maildrop.shtml
http://www.courier-mta.org/maildrop/maildropfilter.html

Environment variables
The following snippet of code is a good opening for your filter script and will set some important variables for your script to use:

Ensure delivery of messages
To make sure unfiltered messages get delivered, make sure to finish your filter with a line to deliver any unfiltered messages. If you don't, the messages will be quietly discarded! There's 2 options you can use depending on where the filter is being called from:

• Use this option to close out a user's mail filter and deliver the message to their Inbox. If you use this in a global filter, called via .qmail-default, a user's .qmail file won't be processed.
  
  to "$VHOME/Maildir"
• For a global filter, called from .qmail-default that will apply to all users, this is the best choice to use. The server will deliver the message using /home/vpopmail/bin/vdelivermail ensuring that a user's own .qmail file will be processed.
  
  to "$VPOP"

Filter Installation
To call a mailfilter, create a /home/vpopmail/domains/<domain>/<user>/.qmail file, containing:

World-read permissions must not be enabled on that file, so we'll adjust it to the vpopmail user:

The .mailfilter file must also not be world-readable:

Logging
Take advantage of our use of Multilog for logging maildrop actions by adding a logfile entry in your filter: